bearing pressure calculation Rev 3.pdf

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Federation of Piling Specialists

CALCULATION OF TRACK BEARING PRESSURES FOR PLATFORM DESIGN

This document sets out the basic procedure for calculating the track bearing pressures for a crane orpiling rig for use in the working platform design process set out in the BRE Report BR470, WorkingPlatforms for Tracked Plant: good practice guide to the design, installation, maintenance and repair ofground-supported working platforms.

It should be noted that the track bearing pressures calculated by the appropriate method for use in theBRE design method are commonly much higher than given by a simple calculation of the total rigweight div ided by the total track area.

The calculation of the track bearing pressures in an appropriate manner consistent with the BRE designmethod is a fundamental requirement of its use, and the use of bearing pressures calculated by anyother means could potentially lead to an unsafe working platform design.

Al l of the figures below are general examples. Platform designs must be based on the actual rigloadings supplied by the piling contractor and the FPS can take no responsibility for any use made ofthe example information shown.

BEARING PRESSURE DESIGN PROCESS

1. Introduction

The calculation of the bearing pressures under the tracks of a piling rig or crane requires a numberof stages.

The first stage is to calculate the theoretical bearing pressure. Using the weights of the variouscomponents, for example the undercarriage, main body, counterweight, mast etc. and theeccentricity of the component from the centre of rotation, the overturning moment can becalculated. This needs to be done for the range of operations that will be carried out, e.g. standing,travelling, handling, penetrating, extracting, with all possible jib or mast orientations considered.

Then, by applying the calculated overturning moments to the section provided by the trackgeometry, the theoretical bearing pressures can be calculated.

This process has to be carried out for all types of envisaged operations and orientations of the rigor crane mast relative to the direction of the tracks.

2. Calculation of theoretical bearing pressures

The factors that need to be considered are broadly:a) Weight and dimensions of rig / crane components, e.g. mast assembly, base machineb) Moment arm to centre line of rotationc) Forces due to line pull, crowd forcesd) Use of stabilising mast foot or outriggers, if any

For convenience, the information required and the basic calculation of the moments generated bythe various rig components or forces involved, can be set out as in the following example.


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FEDERATION OF PILING SPECIALISTSCALCULATION OF TRACK BEARING PRESSURESFOR PLATFORM DESIGN

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Schedule of Rig Components

Rig Manufacturer: Digger Cranes Rig Type: DC 007

Operation Mode: CFA

Completed by:ABC 01/01/05 Checked by:DEFMAIN COMPONENTS

ITEM Weight (kg) MomentArm(m)

Moment (kN.m)

UPPER WORKS Mast assembly 5,600 2.74 150

LOWER WORKS Base machine 21,700 -0.55 -117

SUSPENDED Auger 5,000 3.41 167

EQUIPMENT Rotary head 2,150 3.41 72

COUNTERWEIGHT Counterweight 4,000 -2.45 -96

Information is also required to define the geometry of the tracks and any bearing pads that can be

lowered to improve stability.

Track ground bearing length (m) 3.814

Track pad width (m) 0.700

Distance between centrelines o f t racks 3.300

Front foot pad bearing area (sqm) 1.500 Dimensions: 1.2m x 1.25m

Front foot pad maximum load (kN) 450 Shape Rectangular

Front foot pad moment arm (m) 2.74 Moment (kNm) 1,233

Rear foot pad bearing area (sqm) 0 Dimensions: None

Rear foot pad maximum load (kN) 0 Shape None

Rear foot pad moment arm (m) 0 Moment (kNm)

Finally, the forces or loading that may be applied to the rig or crane need to be defined, reflecting arange of operating conditions to be considered.

Load Force (kN) MomentArm(m)

Moment(kN.m)

Maximum extraction force 392 4.000 1,568

Maximum Line pull 10 4.000 40

Maximum penetration force 29 4.000 116

Maximum auxiliary force 10 4.000 40

These factors need to be considered for the various modes of operation, e.g. travelling, lifting,

drilling, extracting casing etc. The BRE process considers the various loading conditions under twoclass headings.

CASE 1 LOADINGThese loading conditions may apply when the rig or crane operator is unlikely to be able to aidrecovery from an imminent platform failure.

Operations in which this type of loading condition applies could include:

Standing

Travelling

Handling (in crane mode, e.g. lifting a precast concrete pile into the leader, handlingcasings and reinforcement cages)

CASE 2 LOADING


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These loading conditions may apply when the rig or crane operator can control the load safely, forexample by releasing the line load, or by reducing power, to aid recovery from an imminent platformfailure.

Operations in which this type of loading condition applies could include:

Installing casing

Drilling

Extracting an auger

Extracting casing

Rig travelling or slewing with a fixed mast which has a foot or fixed load (e.g. pile heldin the leader) close to the platform surface.

Also, as the rig / crane may be able to operate with the direction of its mast ranging betweenparallel and perpendicular to the axis of the tracks, all possible orientations must be considered.

The process of calculating the theoretical pressure under the tracks should follow the basic

principles set out in EN 791:1996 Drill rigs safety and EN 996:1996 Piling equipment safetyrequirements. The net moment due to the various weights and forces involved is applied to thearea of the tracks in order to calculate the maximum and minimum pressures on each of the tracks.

These pressure distributions may be either rectangular, triangular or trapezoidal, and notnecessarily the same pressure will be present under each track.

It may be found that for some loading conditions, the loaded length for each track may be smallerthan the full track length that could be in contact with the ground.

3. Calculation of design bearing pressures

The calculated non-uniform theoretical loading can be transformed into equivalent uniform loading usingthe method described by Meyerhof (1953).

Total track load W = (P1 + P2).Lg/2

Eccentricity of load centroid Le = Lg.(P1 + 2.P2)/(3.(P1 + P2))

Equivalent uniform bearing pressure for use in design Q = W/(2.Le)

Equivalent track bearing length for use in design L = 2.Le

>>>>>>

W WLe Le

Q

P1

P2

Lg


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4. Example Calculation

The following example shows how a design process may be set out, to calculate how the appliedloading produces a range of equivalent bearing lengths and pressures, for use in the BRE designmethod. This process needs to be carried out for each activity envisaged, and the critical combinationsfound to give the required minimum platform thickness.

It should be noted that this method assumes that the pressures beneath the mast foot and tracks are equaliseddue to redistribution and the validity of this should be checked for each individual case.

5. Typical Operational conditions

PULLDOWN FORCE

TOTAL WEIGHT ON TRACKS IS :-

BASE MACHINE + COUNTERWEIGHT + MAST ASSEMBLY

+ ROTARY TABLE + AUGER

ROTARY TABLE

FOOT PAD CYLINDER

BASE MACHINE

COUNTER WEIGHT

AUGERMAST RAMS

MAST

STANDING / TRAVELLING

FOOT PAD CYLINDER


BASE MACHINE + COUNTERWEIGHT + MAST ASSEMBLY

- FOOTPAD CYLINDER FORCE -PULLDOWN FORCE

AUGER

PULLDOWN FORCE

ROTARY TABLE

COUNTER WEIGHT

BASE MACHINE

MAST RAMS

MAST

DRILLING


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BASE MACHINE + COUNTERWEIGHT + MAST ASSEMBLY- FOOTPAD CYLINDER FORCE + EXTRACTION FORCE

FOOT PAD CYLINDER

AUGER

PULLDOWN FORCE

ROTARY TABLE

BASE MACHINE

COUNTER WEIGHT

MAST RAMS

MAST

EXTRACTING

This guidance has been produced by the Federation of Piling Specialists It is not intended to be used as a design method.Platforms should be designed according to the actual rig loadings supplied by the piling contractor. If expert assistance isrequired, the services of a competent professional should be sought. Although every effort has been made to check the

accuracy and validity of the above guidance, neither the authors nor the Federation of Piling Specialists accept anyresponsibility for mis-statements contained herein or misunderstandings arising herefrom.

September 2005www.fps.org.uk

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